It has been known to provide a traction device in which a single wheel has been coupled in adjustable manner to a worm gear so that by rotation of the worm gear driving rotation of the ground engageable wheels can be effected with considerable mechanical advantage.
Problems have been experienced with such apparatus in that to a certain extent is is difficult to provide the correct gearing for every instance so that in one instance the gearing is too slow for effective operation and in another instance, for instance a large caravan up a steep slope, both traction and the gearing advantage may be too small.
It is also known to provide belts and cogs by which a ground engageable wheel can be driven with mechanical advantage but once again such an arrangement is both cumbersome and offers the same general problem as the worm drive in that it is difficult to provide any effective way of changing the gear ratios and also provide apparatus which is convenient and economic in use.
In another instance, to achieve additional traction. two wheels in co-axial arrangement have been proposed and driven by a common fixed worm drive connection, but such an arrangement is found to be extremely difficult to steer because there is no simple way in which a differential drive coupling can be effected for both wheels is such a system.